The circuit presented here waters your plants regularly when you are out for a vacation. The circuit comprises a sensor part built using only one op-amp (N1) of quad op-amp IC LM324. Op-amp N1 is configured here as a comparator. Two stiff cop-per wires are inserted in the soil containing plants. As long as the soil is wet, conductivity is maintained and the circuit remains off. When the soil dries out, the resistance between the copper wires (sensor probes A and B) increases. If the resistance increases beyond a preset limit, output pin 1 of op-amp N1 goes ‘low’.
IC1 = LM324 (1/4)
IC2 = NE555
R1, R2 = 10K, R3 = 330R (ohm)
VR1, VR2 = 100K
C1 = 100uf 16v, C2 = 0.1uf
D1 = 1N4007
This triggers timer IC2 (NE 555) configured as a monostable multi-vibrator. As a result, relay RL1 is activated for a preset time. The water pump starts immediately to supply water to the plants. As soon as the soil becomes sufficiently wet, the resistance between sensor probes decreases rapidly. This causes pin 1 of op-amp N1 to go ‘high’. LED1 glows to indicate the presence of adequate water in the soil. The threshold pin at which the output of op-amp N1 goes ‘low’ can be changed with the help of preset VR1. To arrange the circuit, insert copper wires in the soil to a depth of about 2 cm,keeping them 3 cm apart. When the soil the water. LED1 glows up as the water reaches the probes.
For small areas a small pump such as the one used in air coolers is able to pump enough water within 5 to 6 seconds. The timing components for IC2 are selected accordingly. The timing can be varied with the help of preset VR2. The circuit is more effective indoors if one intends to use it for long periods. This is because the water from reservoir (bucket, etc.) evaporates rapidly if it is kept in the open. For regulating the flow of water, either a tap can be used or one end of a rubber pipe can be blocked using M-seal compound, adjust VR1 towards ground rail until LED1 turns off and relay RL1 is energized. The motor starts pumping turned along its length to water several plants.